What I learned from my first tour which was an overnighter, was to check to make sure the route that you have planned are A) the roads are paved from start to end, B) the topography of the roads your going to be riding.
I did not verify that the neighboring county had their part of a county road was paved and I ended up riding and walking through 4 miles of loose dirt. It was not really as bad a constantly riding up and down long and some steep hills. The time I got to a campground, all I wanted to do was sleep.
The most important thing I learned from that trip was that I really loved bike touring. I have been on numerous overnight trips, almost every other weekend. Take your time, and enjoy your trip.